The present invention relates to fiber optic networks, and more particularly to wavelength division multiplexers, wavelength division de-multiplexers, optical isolators and optical circulators utilized in fiber optic networks carrying wavelength division multiplexed information signals.
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this specification, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
The term xe2x80x9cwavelength,xe2x80x9d denoted by the Greek letter X (lambda) is used herein in two senses. In the first usage, this term is used according to its common meaning to refer to the actual physical length comprising one full period of electromagnetic oscillation of a light ray or light beam. In its second usage, the term xe2x80x9cwavelengthxe2x80x9d is used synonymously with the terms xe2x80x9csignalxe2x80x9d or xe2x80x9cchannel.xe2x80x9d Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, xcex, and a plurality of n such channels are referred to as xe2x80x9cn wavelengthsxe2x80x9d denoted xcex1-xcexn. Used in this sense, the term xe2x80x9cwavelengthxe2x80x9d may be understood to refer to xe2x80x9cthe channel nominally comprised of light of a range of physical wavelengths centered at the particular wavelength, xcex.xe2x80x9d
A crucial feature of fiber optic networks is the separation of the composite optical signal into its component wavelengths or channels, typically by a wavelength division demultiplexer. This separation must occur to allow for the exchange of signals between loops within optical communications networks. The exchange typically occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths. Conventional methods utilized by wavelength division de-multiplexers in separating a composite optical signal into its component channels include the use of filters and fiber gratings as separators. A xe2x80x9cseparatorxe2x80x9d or xe2x80x9cchannel separatorxe2x80x9d, as used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels of a composite optical signal from one another, or else combines separate channels from separate inputs into a single composite optical signal.
A problem with the conventional separators is the precision required of a transmitter devicexe2x80x94that is, a light emitting device which generates an optical signal at a particular wavelengthxe2x80x94because of the narrow pass bands of such separators. This high precision is difficult to accomplish. Further, signal transmitting devices must be aligned individually for each separator, which is time intensive. Still further, additional, separate optical isolator and optical circulator components must supplement the separators within the optical communications system, which is wasteful of space and resources. Also, there does not exist any single apparatus that combines the functionality of a channel separator with that of an isolator, a circulator, or a comb filter.
Accordingly, there exists a need for a multi-functional separator which is easily aligned. The multi-functional separator should be able to incorporate additional isolator, circulator and comb-filter functionalities. The present invention addresses such a need.
The present invention provides a multi-functional separator which may be used as a demultiplexer or as a multiplexer in wavelength division multiplexed optical communication systems. The preferred embodiment of the multi-functional separator includes a first polarization beam splitter, a non-reciprocal rotator; a reciprocal rotator, a second polarization beam splitter, and a non-linear interferometer. Each of the polarizing input and polarizing output ports includes an optical fiber, a collimator, a birefringent walk-off plate and a non-reciprocal optical rotator. The collimator includes a glass plate and a lens/spacer element between the glass plate and the birefringent walk-off plate. The multi-functional separator is easily aligned by adjusting the positions of each of the polarizing input and output port. Further embodiments of the present invention provide additional optical isolation, optical circulation, optical comb filtering and/or two-stage channel separation capabilities.